(REUTERS) - Twitter Inc co-founder Biz Stone said on Tuesday (May 16) he would be returning full-time to the microblogging service in an unspecified role.

"My top focus will be to guide the company culture, that energy, that feeling," Stone said in a blog post, adding he would not be replacing anyone at Twitter when he joins the company in a couple of weeks.

Twitter Chief Executive Jack Dorsey had asked him to come back to work at the company, Stone said in the post.

"Excited to have Biz's energy and heart back at Twitter! #BizIsBack," Dorsey tweeted on Tuesday.

Stone returns to Twitter at a time when the company is doubling down in its efforts to boost user growth.

He was one of the co-founders of Twitter along with Dorsey, Ev Williams and Noah Glass.

Stone left Twitter in 2011 and went on to co-found online publishing platform Medium and search engine company Jelly, which was acquired by Pinterest in March.

"The (Pinterest) deal did not require me to work at the company we sold to, but I'm the type of person who has to keep working," Stone said.

Twitter shares were up 2.4 per cent at US$19.65 (S$27.40).